You actually asked how many rock musicians played Jazz and then the last question implies the question was about Jazz players playing Rock?
Very few rock musicians wanted to change to jazz, nor were most capable of the change if they did want to. Ginger Baker comes to mind as a rock player who actually wanted to play Jazz.
Jazz players played rock because it was the current trend. Maybe about money though, as nobody wants to go hungry. Did Shorter sell his soul with Weather Report? As the different trends go by young players who grew up listening to that trend will pick it up in their music. Lots of hip hop and Rap beats in the current Jazz. Classical always has an effect on the musicians. I remember Parker picking up on Stravinsky.
Jazz has never been stationary.